symmetrical selection of meshes
Yes: Go into Edit mode. Change your transform pivot type (top of screen) to Active. Select all the vertices to the left of and including the leftmost vertices of the left rectangle that is too narrow (all the vertices to the left). Now SHIFT+CLICK any one of the left-most vertices. This will make that vertex &quot;active&quot; (doing this again will deselect it--but you want it to be active).
You are going to scale all of those vertices along the X-axis towards that active vertex.
Type S, X, and drag the mouse a little to the left until you are satisfied with how thick the red region on the left in your image is.
This is not the only way to do what you are tying to do.  You also could have moved the cursor and used that as your transform registration point. You also could have used Proportional Editing.